Recent research in micro-power technology has successfully demonstrated technology with ability to control micro-scale combustion of liquid hydrocarbon fuels. Potential applications of the micro combustion technology include new generations of batteries, portable chemical reactors, and micro robots and rovers. For example, see Femandez-Pello A.Carlos “Micro-Power Generation Using Combustion: Issues and Approaches.” 29th International Symposium on Combustion Jul. 21-26, 2002 Sapporo, Japan. Given the ability to store substantial amount of power in a compact volume, more applications are expected to impact many fields of technology.
In comparison to current state-of-the-art batteries, liquid hydrocarbon has a significantly higher energy density than the highest-capacity battery today. For example, Li-Ion battery has an energy density of 0.5 MJ/Kg while many hydrocarbon fuels can deliver a specific energy density of 45 MJ/Kg With its extremely high energy density, liquid hydrocarbon is heavily favored as a power source for portable power generators.
Many approaches have been taken to harvest energy generated by combustion from liquid fuel for conversion into electricity. These approaches include producing pressurized gas to actuate a micro turbine and generator, heating a thermoelectric generator, and producing thermal radiation for conversion by a photo-voltaic device. Yet another approach to power generation is by using a fuel cell: the combustion heat provides the energy to break down storable gases (such as ammonia) into hydrogen that is subsequently used by the fuel cell to produce electricity.
The energy production capacities of micro-generators have typically fallen in two broad ranges depending on the size of the system. Micro-scale systems (for example, millimeter sizes) have produced power from micro-to milli-watts, while the meso-scale systems (for example, centimeter sizes) have yielded from tens to hundreds of watts. The manufacturing method and complexity are significantly different between the two types of systems: Micro-scale systems are integrated devices manufactured using silicon processing, while meso-scale devices typically involve complex assemblies of miniature components and involve elements that operate at high speeds, such as micro engines.
In reviewing the demonstrated power of various systems, it is apparent that no micro-scale system is capable of producing power around few watts. A power source producing a few watts of power is of great commercial interest because it meets the requirement for portable electronic products such as cell phones, laptop computers and gaming devices. Several hybrid systems, however, have demonstrated near one watt power by combining micro combustion with commercial thermoelectric modules.
While research in micro combustors has made substantial progress the inventor is not aware of any development to date that reported net positive power generation. Mostly, the electrical power needed for operating accessory equipment such as pumps, valves, and mass flow controller are excluded in the power calculation. To yield a net positive power, it would be necessary to develop ultra-low power systems that include, in addition to a fuel-efficient microcombustor, components needed to sustain stable combustion, handle fuel transport evaporation, mixing flow control, interface sensors and electronics. Currently the efficiency of the conversion rate from heat to electricity remains a major issue, but low-power accessory components are also equally lacking. Other challenges for the developer include system manufacturability, reliability, and of course, cost of fabrication.
Multiple approaches are necessary to achieve net positive power generation. First, rigorous thermal management is needed to ensure that heat generated is directed into an electrical generator with minimum loss to the environment. Second, ultra-low power fluid and gas handling components are needed. Third, an efficient method for integrating the combustor with accessory components is needed. Lastly, although not related to the net power issue, it is vital that the resulting micro system can be produced in high volume and at effective cost.